1. Field of the Invention
This invention relates generally to a tire pressure monitoring system and, more particularly, to a tire pressure monitoring system that optimally positions the receiving antenna in the system to increase the performance and reduce the power required by the system.
2. Discussion of the Related Art
All new vehicles manufactured and sold in the United States are required to have a tire pressure monitoring system (TPMS) that monitors the pressure and temperature of the tires on the vehicle. Typically, the TPMS is an active system that includes its own power source, such as a battery, and a processor for processing data. The TPMS includes a suitable pressure sensor that measures the pressure within the tire and a suitable temperature sensor that measures the temperature within the tire. Sensor signals from the pressure and temperature sensors are sent to the processor, which conditions and encodes the data and the encoded signals are then modulated onto a carrier wave to be transmitted by an antenna. The signals transmitted from the TPMS are received by a receiver at a suitable location within the vehicle, where the received data is demodulated, decoded and used to determine whether a warning signal should be given that the tire pressure is too high or too low, or the tire is too hot.
In the system described above, the TPMS does not actually know if the receiver is receiving the signals it transmits. Therefore, various schemes are performed to ensure that the receiver does receive the signal, including broadcasting the tire information in redundant frames of data and at a high enough signal strength. Because of this, the battery that powers the TPMS tends to be discharged at a rate that may be higher than is necessary. When the battery becomes discharged, and the receiver is not receiving the signals, then the receiver issues a warning signal that the battery may be dead. Because of the manufacturing complexities of placing the TPMS within the tire, it is typically more cost effective to replace the tire than replace the TPMS. Therefore, an added cost may be incurred when replacing tires that may not be worn out.
Further, the performance of a TPMS is highly statistical, and is affected by many uncontrolled factors, such as vehicle speed, road surface material, occupant and cargo loading, and electro-magnetic interference (EMI) from on-board and off-board sources. Integration factors include the size of the wheel, construction of the tire, location of the receiver antenna, data encoding used, RF modulation schemes employed, transmitter battery life and different types of EMI.
In known systems, validation of the TPMS signal allows for up to two or three data packets per frame to be dropped per transmission, which gives a 50% to 60% per frame reliability. Also, as the consumer fleet matures to saturation, the ambient noise floor will increase, further degrading the over-all fleet performance. In order to ensure a desired packet error rate (PER), it is necessary to provide a reliable communications link between the TPMS and the receiver, which may include increasing the transmit power. Alternatively, the number of frames in the packet can be increased so that the redundancy of the data in each packet is increased. Therefore, if part of the packet is lost, there is still a good chance that the information in the packet can be deciphered by the receiver. It is also possible to increase the number of packets that are transmitted to better ensure that the receiver receives the information. However, an increase in the number of packet transmissions also increases the power and consumption.
Further, the antenna associated with the receiver is typically part of the receiver unit and is located therewith. The receiver may be in the vehicle trunk or other locations where it is behind vehicle structures, including metal body structures that are part of the vehicle. Thus, signals from the TPMS may be attenuated or prevented from reaching the antenna of the receiver depending on its location.